A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
A projection optics assembly that includes a support frame is known. Conventionally, the projection optics assembly includes a reference frame and a plurality of sensor frames which are mounted in the reference frame. The sensor frame is a frame adapted to support among other components, an optical element, typically a moveable mirror, one or more sensor units for sensing the position of the optical element, and one or more actuators, which is responsive to the sensor unit, for moving the optical element to a desired position. A separate sensor frame is provided for each mirror. Such a sub-assembly is conventionally referred to as a “mirror module”. Conventionally, the approach to designing a support frame is to provide a plurality of mirror modules. The exact number will depend on the particular projection optics assembly. Typically, however, the projection optics assembly includes 6 mirrors. Each mirror module includes a sensor frame that contains all of the necessary components to have a fully functional adjustable mirror. Each of these mirror modules are then mounted into a common reference frame, typically made of a low expansion glass material, such as ZERODUR®. Typically, one of the mirrors in the projection optics assembly is stationary. In conventional systems, this stationary mirror is also mounted on the common reference frame.
Such conventional arrangements have problems. First, in order to mount the components on the sensor frame, a large number of optical spacers are required. In particular, a large number of spacers are necessary to mount a sensing element, which senses the position of the mirror. The use of spacers in the arrangement is expensive, in particular, in terms of the man hours required to mount the spacers in the sensor frame.
Second, it has been found that attaining accurate positioning after mounting the mirror modules in the reference frame can only be achieved using yet further spacers, which adds further to the manufacturing costs and time scales of the support frame.
Third, the mirror modules are mounted into the reference frame using a so-called “statically determined interface”. Such an interface includes elements whose functionality may be compared with a flexible rod, since the interface element is designed to be stiff in one direction, and as compliant as possible in the other five directions. The interface elements are complex constructions, each including a number of parts. Typically an interface element connects two ZERODUR® parts of the support frame construction. For example, one side of the interface element connects with the reference frame, and at the other side, the interface element connects with the sensor frame. The interface elements are attached to respective ZERODUR® frame parts. The attachments are metal inserts. These inserts are glued into the ZERODUR®. In an attempt to minimize thermal problems caused by the inserts, they have a particular design. It has been found that, in spite of the design of the inserts, the fact that metal inserts need to be glued into the two ZERODUR® frame parts to be connected increases the thermal stability problem.
Furthermore, the interface element should allow for manufacturing tolerances of holes formed in the interface elements in order to connect them to the frame. This results in a misalignment, both in position and angle of the two inserts glued into the two parts to be connected. Conventional interface elements should, therefore, also include an element to allow and compensate for this. Also, since the insert is of a metal material, its thermal expansion in a longitudinal direction is non-zero. This longitudinal direction is the stiff direction, that is, the direction that determines the position of the module relative to the reference structure. This position should ideally remain as constant as possible with changing temperature, and should also be compensated for internally. A conventional interface also includes an element to lock the interface element into place once the mirror module is positioned. From the above discussion, it will be understood that the conventionally required interface is highly complex in terms of design and implementation.
A “statically determined interface” is one in which the interface is made such that only six degrees of freedom (DOFs) of a module are constrained (no more and no less). This means that the module is mounted in six degrees of freedom to its environment. In an ideal case, this means that if the environment distorts, for example, due to thermal effects, the suspended module will only displace and/or rotate as a whole, but it will not distort. The less statically determined an interface becomes, due to parasitical stiffnesses, the more distortion of the environment, in this case the reference structure, which may cause internal distortion of the module itself. This is unwanted.
One way of creating a conventional statically determined interface is to design six identical interface elements, which approximate the functionality of a flexible rod as near as possible. These six interface elements should be positioned and oriented in such a way that they are capable of constraining six degrees of freedom of the module, but not less. As discussed previously, however, this is a highly complex design.
For some time, it has been the conventional thinking, in view of the manufacturability, reliability and testability demands, required in a projection optics assembly, to provide a modular design. Inextricably linked with a conventional modular design is the interfacing of these modules. As discussed, the interfacing of parts of a construction with high dynamic demands, such as those required in a projection optics assembly, is with precision mechanics. Thus, it is conventional to provide projection optics assemblies having a modular design and using complex interfaces to interface modules. Ideally, each module is a fully functional, fully tested unit. Therefore, mounting a module onto its appropriate interface should not distort or influence the fully tested unit in any way, both in the short term, during the mounting itself, or in the long term. As discussed, “statically determined interfacing” is one approach. It has been found that one drawback of conventional “statically determined interfacing” is dynamic behavior. Demands associated with the dynamic behavior of a construction are associated with, and derived from its required performance specifications in terms of, for example, positional accuracy, which is derived from specifications regarding optical performance, related to image position and quality.
With respect to conventional arrangements, it has been found that it is no longer possible to design a sensor frame with statically determined interfaces that meet the dynamic demands of the projection optics assembly, within the volume available in the support frame, while leaving room for assembly, access and cabling, among other things, and while minimizing internal distortion.
Further, in modern projection optics assemblies, a first moveable mirror is to be positioned with respect to a second mirror with sub-nanometre accuracy. It is understood that in the conventional system described above, in order to do this, five distances should be accurately known: the distance between the first sensor unit and the first sensor frame, the first sensor frame and the reference frame associated with the first mirror module, the reference frame associated with the first mirror module and the reference frame associated with the second mirror module, the reference frame associated with the second mirror module and the second sensor frame, and the second sensor frame and the second sensor unit. Further problems associated with this arrangement are manufacturing tolerances and position accuracy, since dynamic behavior is associated with interface constructions. It is understood that in order to go from one sensor on one sensor frame to another sensor on another sensor frame, eight mounting positions and four interface constructions should be passed through. Further, as mentioned, each construction part has its own internal inaccuracies. Also, each interface position has its own inaccuracies. So, therefore, the more construction parts, and the more interface positions there are, the smaller the inaccuracy value that is acceptable from each individual error contribution, in order to attain one constant acceptable value associated with the position of one sensor on one sensor frame with respect to another on another sensor frame. This latter value is the total value from all contributors. This value is a functional requirement, as it is derived from functional specifications. The more parts, the more contributors, the smaller the values from each individual contributor may be in order to attain a constant sum, to meet the functional requirements.
It has been found that conventional projection optics assemblies, even those constructed of glasses with low expansion coefficient, such as ZERODUR®, and constructed in such a manner to maximize their rigidity, the errors associated with each of the five distances accumulate to make it difficult to meet the initial sensor position accuracy of the projection optics assembly. This leads to a limited imaging quality.